Analysis and Control of an MMC-HVDC System Under an Unbalanced Grid Condition

Abstract

Modular multilevel converter (MMC)-based high voltage direct current (HVDC) systems have been increasingly researched and installed in recent years. During operation, systems may be subjected to various unbalanced conditions, the single-line to ground (SLG) fault is one of the most common unbalanced faults. When a SLG fault occurs in a three-phase alternating current (AC) system, the double-line frequency positive- and zero-sequence harmonic components will be produced besides the double-line frequency negative-sequence harmonic component. And if the control system cannot suppress these harmonic components, they can propagate into the direct current (DC) side of the HVDC system and cause ripples in DC-line voltage and current as well as other unwanted consequences. Among the proposed controllers in the published papers on this topic, rarely controllers can both effectively suppress the circulating current and completely eliminate ripples in the DC-line voltage and current while still keeping the AC-side current balanced under unbalanced grid conditions. Therefore, this paper first analyses the propagation of harmonic components in a two-terminal MMC-HVDC system under a SLG fault. Then, based on the mathematical analysis, a proposed controller including an inner current controller for controlling the negative- and positive-sequence components and a controller for suppressing the zero-sequence component is designed to eliminate harmonic propagation from the faulty three-phase AC system to the DC side of the HVDC system, resulting in suppressing ripples in DC-line voltage and current. The theoretical analysis and proposed controller are verified by a simulation model in PSCAD platform.